1. Field of the Invention
The present invention relates generally to apparatus and methods for the controlled application of heat to develop a predetermined temperature for fusing thermoplastic items together.
2. The Prior Art
It is well known to fuse thermoplastic pipes together by using a cylindrical thermoplastic coupling containing a heating coil. The coil is energized to provide localized heating at the interface between the pipes and the coupling, raising the temperature at the interface to a fusion temperature that is high enough to fuse the coupling and the pipes together.
The pipes will quickly melt and deform if they are maintained at the fusion temperature for more than a very brief time. Consequently, in order to achieve fusion without melting the pipes it is necessary to raise the interface temperature very rapidly until the fusion temperature has been reached and then to allow the interface to cool before the remainder of the pipes has reached the fusion temperature.
If the heat from the heating coil diffuses uniformly through the interface area, the pipes and the coupling will fuse into a mechanically strong, leakproof connection. However, the heat does not diffuse uniformly if prior art methods are used. Instead, the heat diffuses non-uniformly, resulting in hot spots at which the pipes melt and cold spots at which no fusion takes place. The end result is a weak connection prone to leakage or mechanical failure. Non-uniform heat diffusion is a particular problem when the ambient temperature is low, as in the winter.
Accordingly, there is a need for a reliable way to diffuse heat uniformly throughout the interface between the pipes and the coupling, especially at low ambient temperatures, without melting or damaging the pipes.